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Ionised gas keeps Milky Way’s lights on

By Lisa Grossman

Gas clouds falling into the Milky Way’s starry disc fuel star formation, but it’s not clear whether the clouds come from intergalactic space or the Milky Way itself. If the latter, they may have been expelled by dying stars in the disc and are now falling back towards the disc

(Image: NASA/ESA/A. Feild/STScI)

What keeps the Milky Way’s lights on? Giant gas clouds have been found close enough to home to keep the galaxy ablaze.

About one sun-sized star is born in the Milky Way’s starry disc every year, a process that requires a constant source of fuel. “The gas associated with [star formation in] galaxies would have run out a long time ago, unless there’s a source of gas from the intergalactic medium,” says Christopher Howk of the University of Notre Dame in Indiana.

Now he and Nicolas Lehner, also at Notre Dame, think they’ve found a source in the Milky Way&colon; fast-moving clouds of hydrogen raining down on the galaxy’s disc.

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The hydrogen is ionised, which makes it hard to detect. While atomic hydrogen can emit light, ionised hydrogen cannot. It can, however, absorb light, meaning it can be detected only if it lies in front of a bright background source.

Uncertain distance

Previously, brilliant galaxies in the distant universe called quasars had provided the backlight to detect ionised gas clouds suspected of fuelling new stars in the Milky Way’s disc. But it was impossible to tell how far away the clouds were – they could have been anywhere between the Milky Way and the quasars billions of light years away.

Lehner and Howk used the Cosmic Origins Spectrograph, one of the newest instruments on the Hubble Space Telescope, to observe 27 stars known to be within a vertical distance of 10,000 light years from the galaxy’s disc. About half were obscured by gas clouds, meaning the gas must be within that same distance.

The researchers calculated the mass of the gas clouds to be equivalent to 110 million suns, providing plenty of fuel for new stars in the disc.

Mysterious origin

But the clouds’ birthplace is still a mystery (see illustration). They could be structures that formed soon after the big bang and simply remained outside of any galaxy until falling into the Milky Way relatively recently.

Or they could come from the Milky Way itself. Exploding stars in the galaxy’s disc may have blown material outwards, and that material might now be falling back towards the disc in these clouds.

“I don’t think we know it yet,” Fraternali says. Determining the chemical makeup of the clouds with future ultraviolet telescopes could reveal the answer, since exploding stars create heavy elements that would not be present in “primordial” gas created after the big bang.